Silicon carbide ceramics are used in a variety of industrial applications requiring good corrosion resistance and wear resistance, particularly at elevated temperatures. Polycrystalline silicon carbide is typically solid-state sintered with additions of sintering aids such as carbon and boron, or is formed by a reaction bonding process wherein silicon metal is reacted with a carbon source to form silicon carbide.
However, silicon carbide often does not have sufficient lubricity for many applications. Therefore, the use of silicon carbide in such applications has been limited. One possible solution to this problem is the use of graphite-loaded silicon carbide wherein the graphite provides improved lubricity at elevated temperatures. Because of the lubricity of graphite-loaded silicon carbide, graphite-loaded silicon carbide components are useful in a variety of mechanical applications.
Graphite-loaded silicon carbide components have been produced by reaction-bonding processes and by solid-state sintering processes. For example, a reaction bonding process to produce graphite-loaded silicon carbide components is disclosed in U.S. Pat. No. 4,536,449 by Kennedy et al. and issued on Aug. 20, 1985. Kennedy et al. disclose a process wherein silicon carbide powder is mixed with colloidal graphite powder and graphite particles having a size of 75 to 300 micrometers. The mixture is formed into a green body which is sintered at a temperature of 1400.degree. C. to 1650.degree. C. in the presence of molten silicon which reacts with the colloidal graphite, converting it to silicon carbide.
Graphite-loaded silicon carbide bodies produced by reaction bonding typically have a low density and include approximately 2 to 20 weight percent free silicon metal. For example, Kennedy et al. disclose that a continuous silicon carbide matrix is formed in a substantially continuous free silicon phase. Consequently, it is believed that the components will have poor strength at temperatures near the melting point of silicon (e.g., temperatures of about 1410.degree. C.). In addition, it can be difficult to control the final amount of free graphite in the sintered body due to the reaction of the graphite with the silicon metal.
A direct (solid-state) sintering process to produce graphite-loaded silicon carbide bodies is disclosed in U.S. Pat. No. 4,525,461 by Boecker et al. and issued on Jun. 25, 1985. Boecker et al. disclose a process wherein silicon carbide particles having a maximum particle size of 8 micrometers are blended with graphite powder having an average particle size not in excess of 8 micrometers. Sintering aids such as aluminum, boron, beryllium, or compounds thereof, and either a temporary binder or an organic solvent such as acetone or heptane, are also added to the batch, along with amorphous carbon. After blending, the mixture is dried, shaped and pressureless sintered to produce a graphite-loaded silicon carbide body.
Another process useful for producing graphite-loaded silicon carbide is disclosed in U.S. Pat. No. 4,942,145 by Moehle et al. and issued on Jul. 17, 1990. Moehle et al. disclose a process wherein silicon carbide particles having a maximum particle size of 5 micrometers are blended with graphite particles having a particle size of less than about 100 micrometers and preferably from about 0.1 to 10 micrometers. A polysilazane binder and an inorganic solvent are blended with the silicon carbide and graphite. After drying, the mixture is shaped and pyrolyzed at 1200.degree. C. to 1450.degree. C. to produce a graphite-loaded silicon carbide body. In a disclosed example, the sintered body has a density of 2.18 g/cc and a bending strength of 15.9 kg/mm.sup.2 (22.6 ksi).
U.S. Pat. Nos. 4,690,909 and 4,701,426, both by Okuno et al., issued on Sep. 1, 1987 and Oct. 20, 1987, respectively. These patents disclose a process for making a silicon carbide and graphite composite material by adding carbon black to silicon carbide. The carbon black is converted into graphite during sintering wherein the resulting graphite exists as a secondary phase segregated along the grain boundaries of the silicon carbide grains. It is disclosed that the average grain size of the graphite is not more than about 3 micrometers.
Graphite-loaded silicon carbide components produced by direct sintering processes disclosed in the prior art are believed to have insufficient lubricity for many applications. In such applications, insufficient lubricity will not adequately reduce the coefficient of friction between the component and the mating surface to prevent the generation of excessive amounts of heat when the component is in motion. Excessive heat generation can cause the component to fail prematurely.
There is a need for a graphite-loaded silicon carbide component having a high degree of lubricity. There is a further need for a graphite-loaded silicon carbide component having good sintered density and strength. There is a further need for a graphite-loaded silicon carbide component that maintains good strength at elevated temperatures. There is a further need for a graphite-loaded silicon carbide component having good lubricity that is substantially free of unreacted silicon.